It is possible for such an emulsion to be “broken”, i.e. for it to lose its characteristics as an emulsion by various mechanisms such as creaming, flocculation or coalescence. These phenomena, which sometimes lead to the destruction of the emulsion, are particularly sensitive to the size of the particles in suspension and to the viscosity of the external phase. The finer the particles of the internal phase and equally, the more viscous the external phase, the stabler the emulsion will be and vice versa. In a way, the stability is a measurement of the time that said emulsion takes to lose its characteristics as an emulsion.
In many fields, such as cosmetics or pharmaceuticals, products in the form of an emulsion must meet preservation standards in regards of microbial proliferation. One of the methods for ensuring this preservation is to include preservatives in the product, such as esters of 4-Hydroxybenzoic acid, sorbates or glycol esters. These additives take no part in the active substances required of the preparations that contain them; nevertheless they can be poorly tolerated. Therefore, it is preferable to propose products, in particular in the dermocosmetics field, that do not use such preservative additives, but whose shelf life and microbial cleanliness are adequate. To this end, said products must be sterile. Sterility is defined by standard EN 556 and the European Pharmacopoeia in force as the probability of a microorganism proliferating in said product. Typically, said probability for a sterile product is below 10−6. The applicant has determined that for products targeted by the invention, sterilization must be performed according to a method with a sterilizing value, F0, equal to 22 minutes. This value F0 gives a time, expressed in minutes, that quantifies the lethal effect of humid heat at 121° C. on viable microorganisms; the method for determining it is defined by the European Pharmacopoeia in force. A product that is sterile according to the definition of the European Pharmacopoeia has been subjected to a sterilization method with a sterilizing value F0 equal to at least 15 minutes. In practice, said method may comprise several cumulative sterilization steps that do not use only humid heat, but whose cumulated lethal effects are equivalent to this value F0.
The lethal effect is measured in relation to a reference germ: sporulated geobacillus stearothermophilus. These bacteria are particularly heat-resistant and heat-tolerant. Thus, the target sterilization effect is that which would have been produced on these germs, in the medium under study, by heat treatment at 121° C. for 22 minutes. The time for this treatment increases exponentially as the temperature decreases, depending on the type of microorganism that the treatment is to destroy. Consequently, the following formula yields the value of F0:F0=t·10(T-121/z) 
where:                t is the treatment time expressed in minutes;        z is a temperature scale and is defined by the heat resistance of the microorganism under consideration. The value of z is defined experimentally with regard to a parameter D. D is a decimal reduction time, which measures the time required at a given temperature, here 121° C., to reduce the concentration of the microorganism under consideration by 90%. D equals one minute for geobacillus stearothermophilus. Thus, z is the temperature variation that changes the value by a factor of 10; for geobacillus stearothermophilus, z equals 10° C.; these factors, D and z, depend on the medium and in particular vary according to the type of emulsion;        T is the treatment temperature.        
Thus, a treatment with a sterilizing value equal to 22 is a treatment that is 22 minutes long at 121° C. (394 K), or a treatment that is 36 seconds long at 135° C. (408 K).
Of course, heating in this way, which reduces viscosity of the emulsion's external phase considerably, has a destructive effect on this last, such that heat sterilization treatments are proscribed by experts and the use of additives or other methods is preferred. These other methods, however, also have disadvantages or are not applicable:                sterilization by irradiation is difficult to implement and there are doubts regarding the decay products;        sterilization by filtering membrane is not, in general, suitable for products as viscous as those used in dermocosmetics; in addition, it may retain the internal phases when these are in condensed or solid state. Lastly, it does not support continuous processes or flow-rates compatible with industrial-scale production of the product.        
Document EP-B-2032175 describes a sterilization method, called UHT or Ultra High Temperature, which can be adapted to the sterilization of cosmetic products such as those targeted by the invention. The method and the device described in this document use indirect heating of the emulsion, which is conveyed while kept under pressure in heating and cooling baths, through a tube. This tube ensures there is no contact whatsoever between the products and the baths mentioned. This type of method induces a thermal gradient between the product in contact with the walls of the tube and the product at the center of the flow, and also a thermal gradient between the start and end of the transit through the same bath. These characteristics make it difficult to check the lethal value of the sterilization thus performed. Therefore, with this method it is not possible to control strictly the durations of heating, plateauing at high temperature and cooling. In fact, the thinness of said tube makes it difficult to achieve adequate flow-rates, particularly when the product to be sterilized is very viscous and because these durations are controlled by the flow-rate of the product through the tube. In addition, the heating and cooling of the product during its travel into the tube change its viscosity, both along the tube and through its cross section; this makes controlling the pumping of said product complicated and requires the use of high pressures. Furthermore, contact with the hot walls of the tube is likely to affect the quality of the product negatively, whereas contact with said tube's cold walls, in the cooling zone, has a tendency to cause the fatty substances within said product to solidify with no possibility of subsequent re-homogenization. Controlling this method is therefore a delicate matter and the results poorly reproducible. Lastly, this method is not suitable for industrial-scale treatment of high volumes of product, with flow-rates above 1 m3/hour.
Other ultra-high temperature treatment methods known from prior art, in particular by infusion or injection, as applied to food products, are also not very suitable for processing this type of product, and require the contact with steam to be realized when the product is in the liquid phase in order to achieve full effectiveness. Even though the sterilization properties required for food products are less stringent than for the products targeted by the invention and even though the organoleptic properties of food products that are treated using ultra-high temperature sterilization methods are less fragile than the stability of the emulsions targeted by the invention, it is common, according to prior art, to add surfactant additives to said products to strengthen their stability in regard of the sterilization cycle. Thus, documents EP 0524751 and JP 2004 105181 describe methods of sterilizing food emulsions, which food emulsions are stabilized beforehand by adding an ester. This type of surfactant additive is similar to preservatives, which the invention aims to eliminate.